Holography with the Wi-Fi-router

Scientists at the Physics Department and Walter Schottky Institute of TUM have developed a
holographic imaging process that depicts the radiation of a Wi-Fi transmitter to
generate three-dimensional images of the surrounding environment. Industrial
facility operators could use this to track objects as they move through the
production hall.

A cross made of aluminum foil between the viewer and the WLAN-router can easily
be reconstructed from the WLAN-hologram as can be seen in the inserted picture
– Image: Friedemann Reinhard/Philipp Holl / TUM

Just like peering through a window, holograms project a seemingly
three-dimensional image. While optical holograms require elaborate laser
technology, generating holograms with the microwave radiation of a Wi-Fi
transmitter requires merely one fixed and one movable antenna, as Dr.
Friedenmann Reinhard and Philipp Holl report in the current issue of the
renowned scientific journal Physical Review Letters.

“Using this technology, we can generate a three-dimensional image of the
space around the Wi-Fi transmitter, as if our eyes could see microwave
radiation,” says Friedemann Reinhard,
director of the Emmy Noether Research Group for Quantum Sensors at the
Walter Schottky Institute of the TU Munich. The researchers envision
fields of deployment especially in the domain of industry 4.0 –
automated industrial facilities, in which localizing parts and devices
is often difficult.

Wi-Fi penetrates walls

Processes that allow the localization of microwave radiation, even
through walls, or in which changes in a signal pattern signify the
presence of a person already exist. The novelty is that an entire space
can be imaged via holographic processing of Wi-Fi or cell phone signals.

“Of course, this raises privacy questions. After all, to a certain
degree even encrypted signals transmit an image of their surroundings to
the outside world,” says the project leader, Friedemann Reinhard.
“However, it is rather unlikely that this process will be used for the
view into foreign bedrooms in the near future. For that, you would need
to go around the building with a large antenna, which would hardly go
unnoticed. There are simpler ways available.”

Centimeter-scale precision

Hitherto, generating images from microwave radiation required
special-purpose transmitters with large bandwidths. Using holographic
data processing, the very small bandwidths of typical household Wi-Fi
transmitters operating in the 2.4 and 5 gigahertz bands were sufficient
for the researchers. Even Bluetooth and cell phone signals can be used.
The wavelengths of these devices correspond to a spatial resolution of a
few centimeters.

“Instead of a using a movable antenna, which measures the image point by
point, one can use a larger number of antennas to obtain a video-like
image frequency,” says Philipp Holl, who executed the experiments.
“Future Wi-Fi frequencies, like the proposed 60 gigahertz IEEE 802.11
standard will allow resolutions down to the millimeter range.”

Looking to the future

Well-known optical methods for image processing can also be deployed in
Wi-Fi holography: One example is the dark-field methodology used in
microscopy, which improves the recognition of weakly diffracting
structures. A further process is white-light holography in which the
researchers use the remaining small bandwidth of the Wi-Fi transmitter
to eliminate noise from scattered radiation.

Simulation of a warehouse: from the “light” of the WLAN router in the basement,
the three-dimensional image (right) can be reconstruced .
– Image: Friedemann Reinhard/Philipp Holl / TUM

The concept of treating microwave holograms like optical images allows
the microwave image to be combined with camera images. The additional
information extracted from the microwave images can be embedded into the
camera image of a smart phone, for example to trace a radio tag attached
to a lost item.

But the scientists are just at the beginning of the technological
development. For example, research on the transparency of specific
materials is lacking. This knowledge would facilitate the development of
paint or wall paper translucent to microwaves for privacy protection,
while transparent materials could be deployed in factory halls to allow
parts to be tracked.

The researchers hope that further advancement of the technology may aid
in the recovery of victims buried under an avalanche or a collapsed
building. While conventional methods only allow point localization of
victims, holographic signal processing could provide a spatial
representation of destroyed structures, allowing first responders to
navigate around heavy objects and use cavities in the rubble to
systematically elucidate the easiest approach to quickly reach victims.

The research was funded by the Emmy Noether Program of the German
Research Foundation (DFB) and the TUM Junior Fellow Fund.